Within the brewing industry, yeast used in a fermentation is commonly recovered from the fermentation and used again in several subsequent fermentations. It is imperative that the yeast remains free of bacterial contamination. One way of preventing bacterial contamination is to treat the yeast slurry to selectively eliminate bacteria (particularly beer spoilage lactic acid bacteria) under conditions that permit survival of the yeast cells. In one method, harvested brewers yeast is treated with acid to lower the pH to a point at which bacteria die and at which the yeast survive. Typically, inorganic acids (e.g., sulfuric or phosphoric acid) or organic acids (e.g., lactic acid) have been used for this disinfection process. However, our research has surprisingly shown that when the yeast has not been exposed to hop acids, as in the case of post fermentation hopping, acid disinfection is ineffective. This observation led to the unexpected discovery that hop acids (e.g., isohumulone) used in combination with phosphoric, sulfuric or lactic acid greatly increases the bactericidal effectiveness of the acid treatment.
However, there exist certain acid tolerant isolates of lactic acid bacteria such as Pediococcus and Lactobacillus species that are resistant to acid treatment, even when hop acids are present in harvested yeast taken from a kettle hopped fermentation or are added to yeast at concentrations typically found in kettle hopped fermentation.
Therefore, there exists a need in the art for a method of decontaminating yeast that is effective at reducing acid-resistant strains of lactic acid bacteria.
The present invention describes, inter alia, the use of an isomerized, reduced, and hydrogenated hop acid, tetrahydroiso-alpha acids (tetrahydroisohumulone, tetrahydroisocohumulone and tetrahydroadhumulone), in combination with organic and/or inorganic acid to disinfect brewer's (brink) yeast from all contaminant bacteria including acid resistant lactic acid bacteria. This combination is surprisingly more effective at killing lactic acid bacteria than inorganic or organic acids in combination with isohumulone (which is the primary alpha acid in kettle hopped beers) or in combination with reduced iso-alpha acids (dihydroiso-alpha acids). Tetrahydroiso-alpha acids are typically used as a post-fermentation hopping agent for adding bitterness and foam stability to beers and, as such, is not present in yeast harvested from fermentation. The addition of this compound at low concentrations to brink yeast prior to acid treatment has been shown to greatly increase the effectiveness of acid treatment for killing lactic acid bacteria. Raising the concentration to 40 ppm of tetrahydroiso-alpha acids in brink yeast is surprisingly very effective at killing even acid resistant bacteria. Furthermore, adding 40 ppm tetrahydroiso-alpha acids to brink yeast during storage and prior to acid treatment acts as a strong antimicrobial agent that reduces bacteria prior to acid treatment without having any appreciable effect on the yeast.